CdS Quantum Dot Pt Nanoparticle Aerogel Composites for Photocatalytic Hydrogen Evolution—Consequences of Composite Preparation Method on Photocatalytic Activity

Developing more active and stable photocatalysts is fundamental for the improvement of photocatalytic water splitting reaction, and consequently for the production of green hydrogen. Hydrogen is estimated to be the main future fuel, releasing three times more energy upon combustion than current fossil fuels, and water as waste. One major bottleneck for the establishment of hydrogen as fuel is the lack of efficient production methods. Photocatalytic water splitting reaction enables the obtaining of hydrogen from the light-driven catalytic breakage of water into its elemental components, H₂ and O₂, and its efficiency is dictated by the photocatalyst used.
Pt-CdS composites are among the most prominent systems to be used as photocatalysts for water splitting. CdS is the most studied visible-light photosensitizer, harvesting photons in the visible range that compose the majority of the sunlight spectrum irradiance. Pt is the benchmark metal to be used as co-catalyst coupled with photosensitizers to improve charge separation and increase catalytic turnover by improving electron transfer rates to reduce water. Seeking to design an improved photocatalyst, we have reported oxidatively assembled CdS quantum dots aerogels as very active catalysts for water reduction, producing higher H₂ rates than several Pt-CdS systems in literature. This efficiency comes from a combination of: (I) the quantum confinement effect of quantum dots (QDs), that enables the tuning of the material band gap with the particle size, tuning the photons absorption range to higher energy visible photon; (II) high surface areas derived from the mesoporous aerogel framework obtained with sol-gel processing of QDs; and (III) exciton delocalization effect caused by the bonding formed between the QDs in the gel framework, that improve charge separation and hinder charge recombination.
In this work, we prepared CdS QDs aerogels decorated with Pt nanoparticles (NPs) as photocatalysts for water splitting reaction. We hypothesized that combining the beneficial aspects of the CdS QD gel framework with Pt NPs would further improve the efficiency of this system for H₂ production, by exploiting the metal-semiconductor heterojunction for charge separation and the higher catalytic activity of the noble metal. We prepared composites with different synthetic methods, co-gelation and wet impregnation of Pt NPs, and found a strong correlation between the materials structure and their catalytic properties. We will evaluate the disposition of Pt NPs in the gel network with 3D electron tomography, analyze the degree of Pt NP-CdS QDs connections using Graph Theory calculations. The degree of connectivity will be correlated with the materials photophysical characterization data obtained by ultrafast time-resolved spectroscopy techniques to evaluate mechanistic steps involved in the photocatalytic reaction.